Photoconductive target



Feb. 6, 1962 J. F. NlcHoLsoN ETAl. 3,020,442

PHoTocoNDuc'rIvE TARGET Filed may 11, 1959 -Illli- 42 Amorphous Selenium WITNESSES F INV'ENITORSB a, James .Nic o son t W Kenneh R. Simpson rates arent ire atented Feb. 6, 1&62

3,020,442 PHOTGCGNDUC'HVE TARGET .lames F. Nicholson and Kenneth R. Simpson, Elmira, NY., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation ot Pennsylvania Filed May 11, 1959, Ser. No. 812,226 5 Claims. (Cl. 315-16) This invention relates to photoconductive devices and more particularly to a photoconductive target for a pickup tube.

One particular application of this photoconductive target is in a pick-up tube known as a vidicon. The vidicon pick-up tube comprises an evacuated envelope of a suitable material. The envelope is providedy with a face plate portion transmissive to input radiation. An electrically conductive film is provided on the inner surface of the face plate. The conductive coating is the signal plate or back plate of the target and a layer of photoconductive material is provided on the conductive film to complete the target member. An electron gun is provided at the opposite end ofthe envelope with respect to the face plate for scanning the target member. The cathode of the electron gun is at a negative potential with respect to the signal plate of the target. The electrons within the scanning beam may be of high energy, that is above the -rst crossover potential and below the second cross-over potential of the target. The electron beam may also be of the low velocity type scan in which the energy of the electrons is below the iirst cross-over potential of the target. The most common type of operation is the utilization of the low velocity type scan. Our invention is directed to a target structure for use in the low velocity type scan operation.

In the operation of the low velocity type scan vidicon type-pick-up tube the electron beam is scanned across the target and electrons are deposited on the surface of photoconductive material of the target so that the surface is charged to the potential of the cathode of the electron gun. The back plate is maintained at a positive potential with respect to the cathode of the electron gun. When light is then focused upon an area of the target, it renders the photoconductive material conductive in that particular area and causes a corresponding portion of the scanned surface of the photoconductive layer to discharge toward the potential ofthe back plate. T he next time the electron beam scans this area it again restores or charges the surface to the cathode potential. The return of the area to cathode potential restores the original voltage diderence across the photoconductive layer and causes an electron current to flow in the signal plate. This electron current which flows in the signal plate is coupled to an output circuit to derive an electrical signal representative of and corresponding to the light directed on the area of the photoconductive layer.

The scanning rate in a conventional vidicon utilized in the television industry is such that each element is read by the scanning electron beam thirty times every second. Several different types of photoconductive materials have been used in this application such as selenium and antimony trisuliide. These materials have operated satisfactorily in television applications where the scanning rate is thirty times per second. ln other applications, however, it is desirable to scan at a slower rate such as ten times per second or less. This places certain requirements on the target structure that present devices cannot cope with. In general, the target must have the ability to retain a charge during the time the electron beam is scanning the remaining elements of the target. The target must also have a high dark resistance to pre-r vent leakage and decay `of the signal during the time the electron beam is scanning the remaining elements and the lateral leakage of a target must also be of a very low value to prevent degradation or resolution while the storage and reading operations are performed. The advantage of using a slow scanning rate in a pick-up tube in certain applications is to increase the signal to noise ratio of the output signal and also to reduce the required bandpass of the system. The longer the light remains on the target the greater the discharge and therefore the greater the signal output.

lt was found that the antimony trisuliide type surfaces were not desirable for the slow scan applications because of the low sensitivity, high dark current and generally poor uniformity due to difficulties involved in evaporating the material in a gas atmosphere. These characteristics reduce the capabilities of the surface to retain or build up a charge representing a signal during the long frame time of a slow scan operation. Since the light resistance of an antimony trisulfide surface is relatively high, the surface will exhibit only a small positive change in potential when light falls upon it; Since the dark re-sistance of antimony trisuliide is relatively low, a similar positive charge will appear on the surface due to electrons leaking through the surface over a long frame time. The dilerence in potential over the surface will, therefore, be very small thereby resulting in a very low sensitivity target.

Photoconductive targets manufactured from selenium, on the other hand, are very sensitive to light and a large charge can be built up across such a surface. The dark current in the selenium type target is also much lower than the antimony trisuliide and the charge may be retained over a longer frame time. The selenium target has disadvantages, however, which limit its application to the slow scan type operation. The selenium surface is a very unstable surface and at temperature slightly above F. the material tends to become conductive even in the dark. The selenium is normally evaporated to form an amorphous type selenium. It is found that this amorphous form of selenium has a tendency to revert to a crystalline state which has a much lower resistance. It has also been found that in the operation of a selenium target that if the voltage is raised to a critical potential only slightly above the operating range, conducting spots occur which appear as bright spots in the resulting picture. This is believed to be due to local crystallization within the selenium layer. Another disadvantage of selenium isan effect wherein the scanned surface may become positively charged with respect to the signal electrode. This is believed to be due to the secondary emission from the selenium surface exceeding unity, that is, more secondary electrons leaving the surface than land on it. This results in charging the surface of the selenium to the potential of the nearby collector and accelerating electrode which is of the order of a positive 300 volts with respect to the electron gun cathode. This positive charging of the selenium surface causes the dark current to reverse its direction and no signal is obtained from the signal plate in response to light directed thereon.

1t is accordingly an object of this invention to provide an improved photoconductive target. v

lt is another object to provide an improved photoconductive target of high sensitivity for pick-up tubes.

It is another object to provide an improved photoconductive target for use in slow type scanning operations.

These and other objects are effected by our invention as will be apparent from the following description, taken in accordance with the accompanying drawing, throughout which like reference characters indicate like parts, and in which:

FIG. 1 is a view in section of a cathode ray lcamera tube using a photoconductive target according to the teaching of our invention;

FIG. 2 is an enlarged sectional view of the target Shown in FIG. l.

Referring in detail to FIGS. 1 and 2 our invention illustrates a vidicon type pick-up tube. The tube cornprises an evacuated envelope l2. containing an electron gun assembly 20 and a target member 3%. Suitable deiiection and focusing coils are provided around th envelope l2. The envelope 12 includes a face plate por* tion 14 of amaterial such as glass coated on its inner surface with a light transmissive electrically conductive film 32. This conductive lm is of a material such as tin oxide. The lm or coating 32 is the signal electrode or back plate of the target 30 and an electrical lead-in 34 is provided to the exterior of the envelope l2. The lead 34 is connected through a resistor 35 to a voltage source 37. The signal output is derived from the resistor 35. The photoconductive material is deposited as a thin layer 36 upon the electrical conductive iilm 32. In the specific embodiment shown, the photoeonductive material layer 36 is comprised of two separate layers. A layer 42 of selenium is deposited on the conductive layer 32 and a layer 44 of antimony trisultide is deposited on the exposed surface of the selenium layer 42.

Immediately in front of the photoconductive layer 36 and between the target 30 and the electron gun 20 is a conductive mesh 38 which provides a uniform decelerating field for electrons. The mesh 33 is normally operated at a potential of about 250 volts positive with respect to the cathode 22 of the electron gun 20. The back plate 32 is normally operated at a potential of the order of 20 volts positive with respect to -the electron gun cathode. The voltage source 37 provides this potential.

In the specific embodiment shown in FiG. 2, the photoconductive member 35 consists of two layers 42 and 44. The layer 42 is deposited on the electrical conductive layer 32 to provide a layer of amorphous selenium of less than 10 microns in thickness. This layer is formed by evaporating the selenium in a suitable vacuum. The layer 44, which is deposited on the layer 42, is of antimony trisulde of less than l microns in thickness and is formed by evaporating the antimony trisulde in a suitable vacuum.

ln preparing targets in accordance with this invention, a suitable chemically pure selenium material is obtained. 'I'he glass support member having a conductive coating thereon is placed in a closed container capable of being evaporated. A small quantity of selenium, about 60 milligrams, is placed in a suitable boat of a material such is Nichrome. The boat containing the selenium is then inserted into the container andV positioned at a distance of approximately two inches from the target. The system is then exhausted to a pressure of about 10-5 millimeters mercury. The boat is then heated to approximately 300 C. to 500 C. which evaporates the material from the boat onto the target. This heating is continued for approximately minutes until a target thickness of approximately 5 microns is obtained. The evaporator utilized consists of a circular disc which reects light from a constant tungsten light source of 11.3 watts and 2.7 amperes positioned at a distance of about inches from the target. The color of the light reflected from the evaporator after transmission through the target member is observed during the target evaporation process. When a moderately deep red color is observed, the heat is turned otf and the rst layer 42 of the target has been completed. The layer 42 is an amorphous selenium layer having less density than the bulk density of the material.

The system is now opened to the atmosphere and a boat containing the antimony trisulfide material is positioned within the container at about 2 inches from the target and heated to a temperature of 400 C. to 500 C. to deposit the second layer 44 on the target member.

CTI

The heating step is, of course, performed in a vacuum of about 10-5 millimeters of mercury. Again the desired thickness of the layer 44 is determined by the color of the light reflected from the evaporator and after transmission through the target. When the reflected color is deep red, the evaporation is stopped. This is found to provide an adequate thickness of antimony trisuliide and this thickness is about four microns.

lt is found that this combination of materials results in a surface much superior to either material alone. The composite surface produces much greater sensitivity and less dark current than a conventional antimony trisul'ide surface. Under normal television scanning operation, the signal output current is increased and the darl; eurrent decreased to about 3/1900 of its value for the conventional antimony trisultlde target. For slow scan type operation, the increase in sensitivity is even more apparent.

lt is found that the use of the antimony trisulfide layer on top of the selenium layer inhibits the growth of conducting spots or crystal spots within the selenium layer which allows the back plate electrode to be operated at a higher potential with respect to the electron gun cathode than with the conventional selenium surface alone.

lt is also found that the composite surface will witl1- stand higher temperatures Without damage than a conventional selenium surface alone. It is also found that the positive charging of the surface of the target by secondary emission of the conventional selenium type target no longer takes Place within the range of signal electrode voltages commonly used.

It is also found that the lag of such a surface, or percentage of signal current continuing to flow at a given interval after the removal of the exciting light, is as small as for antimony trisuliide. The lag characteristics of the target are good and it is also found that there is less tendency to burn an image into this type of target than in a conventional selenium target when it is subjected to intense illumination. The composite structure also allows one to evaporate both layers in a vacuum and as a result one is able to position the boat at a sutiicient distance from the target to obtain an extremely uniform deposit of material.

While we have shown our invention in only one form, it will be obvious to those skilled in the art that it is not so limited but is susceptible of various other changes and modifications without departing from the spirit and scope thereof.

We claim as our invention:

l. A photoconductive target electrode for a piek-up tube, said target electrode comprising a transparent conductive iilm, a thin tilm of amorphous selenium on one surface of said conductive film, and an antimony trisulfide coating on said film of amorphous selenium.

2. A pick-up camera tube comprising an electron gun means including a source of electrons for providing an electron beam along a path, a target electrode spaced from said gun means and arranged transversely to said beam path, said target electrode including a transparent support plate, a thin conductive film on the surface of said support plate facing said electron gun, a thin film of amorphous selenium on the surface of said conductive ylrn and a thin tilm of antimony trisuliide on the exposed surface of said amorphous selenium.

3. A light sensitive target for an electron discharge device comprising a support member having an electrical conductive layer thereon, a layer of amorphous selenium of a. thickness of less than lOmicrons deposited on said electrical conductive layer and a layer of antimony trisuliide of a thickness of less than l0 microns deposited on the surface of said amorphous selenium.

4. A light sensitive target for an electron discharge device comprising a support member having an electrical conductive layer thereon, a layer of amorphous selenium of a thickness of about 5 microns deposited on said elec- 6 t trical conductive layer and a layer of antimony trisulde potential with respect to the exposed Asurface of said of a thickness of about 4 microns deposited on the surface antimony trisulide film. of said amorphous selenium. l

5. A photoconductive target electrode for a pick-up References Cited in the file of this patent tube, said target electrode comprising an electrical con- 5 UNITED STATES PATENTS ductive film, a thin film of amorphous selenium on one v surface of said conductive film and a layer of antimony glll; ng' trisulde on the exposed surface of said amorphous sele- 2,910,602 Lubszynsk et al Oct 27, 1959 nium lm, said conductive lm operating at a positive 

